| Making Math-Based Models Better
These new discoveries about the nature of rubisco activase offer a
novel explanation of why photosynthesis slows under those adverse regimens.
Too, the findings may enhance the precision and accuracy of today's
math-based models of how plants will react to climate change. Data from
the Arizona investigations can be factored into the models, adding an
important new dimension that may improve the reliability of model-derived
projections. The research also opens the door to new strategies that
could help crop plants sidestep the unwanted, climate-driven influences
on rubisco activase and the negative impacts on rubisco and photosynthesis.
Crafts-Brandner and Salvucci determined that heat and carbon dioxide
work in different ways to thwart rubisco activase. Heat literally unravels
the enzyme, a process known as denaturing. Unraveling renders rubisco
activase unable to fit correctly onto rubisco. Lacking the correct fit,
denatured rubisco activase can't efficiently convert inactive rubisco
to the necessary active form. Denaturing of rubisco activase can occur
at temperatures as low as 89.6°F. But rubisco continues to function
effectively until temperatures reach 131°F, Crafts-Brandner and
Key Energy Source Strongly Affected
High carbon dioxide impedes rubisco activase by altering levels of
its favorite energy source, a high-energy compound called adenosine
triphosphate, or ATP for short. Carbon dioxide levels cause a shift
in the ratio of ATP to a lower-energy compound called adenosine diphosphate,
or ADP. ATP and ADP occur in all living cells, including leaf cells.
In the lab experiments, higher-than-normal carbon dioxide makes ADP
more plentiful than ATP. This shift in the ratio of the two compounds
significantly impairs rubisco activase.
So what can be done to forestall the unwanted effects on rubisco activase?
Crafts-Brandner and Salvucci hope to find or construct genes that could
cue plants to synthesize a more heat-stable rubisco activase. And they
plan to explore other biotech-based options that could enable rubisco
activase to overcome the unfavorable shift in the ATP to ADP ratio that
high carbon dioxide instigates. They're collaborating with Pioneer Hi-Bred
International under the terms of a research and development agreement.
Taking Earlier Discoveries Forward
The Arizona studies build on breakthroughs made by the team of William
L. Ogren, formerly with ARS in Urbana, Illinois, and now retired; Archie
R. Portis, Jr., with ARS at Urbana; and Salvucci, who worked with Ogren
and Portis as a postdoctoral researcher. In 1985, Ogren, Portis, and
Salvucci discovered the existence of rubisco activase and proved that
it activates rubiscoshort for ribulose-1, 5-bisphosphate carboxylase/oxygenase.
Ogren's group was the first to show that rubisco is activated and regulated
by rubisco activase.
In 1990, Ogren received the coveted International Alexander Von Humboldt
Foundation Award. In addition, he was selected for the ARS Hall of Fame
and named to the National Academy of Sciences.
The newer work by Crafts-Brander and Salvucci about the effects of
temperature and carbon dioxide on rubisco activase was documented in
the Proceedings of the National Academy of Sciences, one of the
world's leading scientific journals.By Marcia
Wood, Agricultural Research Service Information Staff.
This research is part of Plant Biological and Molecular Processes,
an ARS National Program (#302) described on the World Wide Web at http://nps.ars.usda.gov.
Steven J. Crafts-Brandner
and Michael E. Salvucci
are with the USDA-ARS Western
Cotton Research Laboratory, 4135 E. Broadway Rd., Phoenix, AZ 85040;
phone (602) 437-0121, fax (602) 437-1274.
"Robust Plants' Secret? Rubisco Activase!" was published
in the November
2002 issue of Agricultural Research magazine.